Chlorophyll-a measures the amount of algae in the water. Too much algae in the water is not good for the Lagoon’s health and so the higher the chlorophyll-a levels, the lower the score. The table below shows that chlorophyll-a scores dramatically declined from 2009–2016 as algae super blooms expanded through the Lagoon. Starting in 2017, some areas of the Lagoon started to improve.
Historically, algae blooms and fish kills would be occasional events during warm summer months. However, the IRL has recently experienced persistent algae blooms covering hundreds of acres and lasting for months, resulting in extensive fish kills, seagrass losses, and marine mammal deaths. Canal discharges also carry toxic algae into areas of the South IRL.
Targets created by the EPA and SJRWMD were considered for the Mosquito Lagoon (ML), Banana River (BRL), North IRL (NIRL), and Central IRL (CIRL) while the EPA and SFWMD targets were considered for the South IRL (SIRL). After a comparison of the criteria from each source, it was decided that a combination of the established targets would be used in the Indian River Lagoon Ecological Health Assessment (bolded data in Table).
The SJRWMD annual median target was chosen because geometric means (EPA recommended target) were typically less protective of the Lagoon than the District’s median targets. For the CIRL, the SJRWMD annual median targets specific to the north and southern regions were selected (Steward, Lasi, and Phlips, 2010). Finally for the SIRL, the SFWMD’s conservative criteria of annual median was used as target for all three sublagoon areas.
Chlorophyll-a targets are maximum annual medians or geometric means requiring that data be at or below the target to meet the requirements. Chlorophyll-a offsets established for each target by year and for the period of record (POR) are positive if they meet or are lower than the target and negative if they exceed the target concentration.
The central tendency (median and geometric mean) was calculated for each station for each year as well as the POR. Offsets from the target data were calculated from the central tendency and data were averaged for each sublagoon annually and for the entire twenty years. The frequency distribution of the data, as shown in the figure, provided guidance to create chlorophyll-a scores.
To learn more about how scores were calculated, to see the targets for each region, and to read other sciencey stuff, check out the 2020 Health Assessment Methodology Report.